lllllllllillllllllllllllHIIIIIIMIIIIIIIIIIII 
B    32 


: 


COLORIMETRIC  TEST 

FOR 

ORGANIC  IMPURITIES  IN  SANDS 

BY 
Duff  A.  Abrams 

Professor  in  Charge  of  Laboratory;^-    ^, 
AND 

Oscar  E.  Harder 

Chemist 


Circular  No.  1 
Structural  Materials  Research  Laboratory 

Cooperation  of 
Portland  Cement  Association 

and 

Lewis  Institute 


Experimental  work  carried  out  in  cooperation  with 
Committee  C-9  on  Concrete  and  Concrete  Aggre- 
gates, of  the  American  Society  for  Testing  Materials. 
Published    by    permission. 


LEWIS    INSTITUTE 

Chicago 
February,  1917 


43 


Colorimetric  Test  for  Organic  Impurities 
in  Sands 


INTRODUCTION.. 

During  the  past  few  years  the  subject  of  impurities  in  sands 
for  mortar  and  concrete  has  been  receiving  rapidly  increasing 
attention  from  testing  engineers  and  inspectors.  Experience 
has  shown  that  on  account  of  the  presence  of  impurities,  certain 
natural  sands  do  not  give  satisfactory  results  when  used  in  con- 
crete, and  many  failures  and  much  inferior  work  have  been 
attributed  to  impurities  in  the  fine  aggregate.  However,  when 
it  comes  to  determining  before  the  sand  is  used  in  construction 
or  subjected  to  tests  in  mortar,  whether  or  not  the  sand  is  suit- 
able for  fine  aggregate,  there  has  been  no  well-established  cri- 
terion regarding  the  impurities.  Practically  all  specifications 
have  required  that  the  sand  be  clean;  the  so-called  dirty  sands 
have  generally  been  in  bad  repute.  Organic  matter  has  been  the 
principal  source  of  trouble,  yet  many  publications  stating  that 
as  much  as  10  per  cent  of  loan  in  sand  was  not  harmful  and 
might  even  increase  the  strength  of  the  mortar,  are  to  be  found 
in  the  engineering  journals. 

An  investigation  initiated  by  Committee  C-9  on  Concrete  and 
Concrete  Aggregates,  of  the  American  Society  for  Testing  Ma- 
terials, showed  the  desirability  of  a  simple  and  reliable  test  which 
would  make  it  possible  to  detect  sands  which  contained  harmful 
impurities  before  they  were  used  in  concrete.  Commercially,  it 
was  also  desirable  to  develop  some  remedial  measure  which  would 
make  it  possible  to  use  sands  which  in  their  present  condition, 
on  account  of  impurities,  are  not  satisfactory  for  concrete. 

Engineers  and  chemists  have  recognized  the  danger  of  using 
sands  which  contained  appreciable  amounts  of  organic  matter 
which  may  come  from  surface  loan  or  other  sources  of  contami- 
nation, and  have  used  various  methods  for  detecting  such  impur- 
ities. A  method  which  has  probably  been  most  used  depends 
upon  the  loss  on  ignition.  Obviously  the  loss-on-ignition  test 
drives  off,  besides  the  organic  matter,  the  water  of  crystalliza- 
tion, hydration,  etc.,  and  the  carbon  dioxide  (C02).  In  certain 
"limestone  sands"  the  loss  on  ignition  due  to  carbon  dioxide 
alone  may  amount  to  more  than  10  per  cent,  and  yet  the  sand  may 
be  entirely  free  from  organic  matter.  The  determination  of  the 
organic  matter  by  combustion  has  little  advantage  over  the  loss- 
on-ignition  method,  unless  a  correction  be  made  for  the  car- 
bonates present.  Methods  which  depend  upon  the  moist  oxida- 
tion of  the  organic  matter  have  some  advantage,  but  are  not 
entirely  satisfactory.  None  of  these  methods  determines  directly 
the  amount  of  organic  matter  in  the  sands.  Most  of  them  deter- 
mine the  amount  of  carbon  by  first  converting  it  into  carbon 
dioxide.  The  organic  matter  is  then  calculated  by  multiplying 

1 

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the  per  cent  of  carbon  by  some  factor,  the  accuracy  of  which  is 
often  questionable. 

The  colorimetric  test  for  organic  impurities  in  sands  represents 
one  phase  of  the  work  which  has  been  done  at  the  Structural 
Materials  Research  Laboratory,  Lewis  Institute,  Chicago,  under 
the  direction  of  the  Committee  mentioned  above.  The  fact  that 
the  sand  has  passed  the  colorimetric  test  does  not  by  itself  prove 
that  the  sand  is  suitable  for  mortar  or  concrete,  since  its  fineness 
and  other  characteristics  must  also  be  considered  as  they  affect 
the  strength.  The  result  of  the  researches,  however,  indicate  that 
it  is  a  reliable  test  for  organic  impurities,  which  are  the  most 
common  source  of  danger  in  a  sand  of  satisfactory  granulome- 
tric  composition. 

It  is  intended  at  this  time  to  give  only  a  description  of  the 
methods  of  making  the  test  so  that  it  may  be  tried  out  further  in 
commercial  and  private  laboratories.  The  experimental  data 
obtained  in  the  course  of  these  studies  will  furnish  the  subject- 
matter  of  a  subsequent  report. 

It  will  be  of  assistance  to  the  Committee  having  this  investi- 
gation in  charge,  if  other  experimenters  will  report  the  details 
of  their  tests  and  observations  to  the  Chairman,  Mr.  Sanford 
E.  Thompson,  136  Federal  Street,  Boston,  Mass.,  or  to  the 
authors. 

The  colorimetric  test  may  be  described  briefly  as  follows: 

A  sample  of  sand  is  digested  at  ordinary  temperature  in  a 
solution  of  sodium  hydroxide  (NaOH).  If  the  sand  contains 
certain  organic  materials,  thought  to  be  largely  of  a  humus 
nature,  the  filtered  solution  resulting  from  this  treatment  will 
found  to  be  of  a  color  ranging  from  light  yellow  up  through 
the  reds  to  that  which  appears  almost  black.  The  depth  of 
color  has  been  found  to  furnish  a  measure  of  the  effect  of  the 
impurities  on  the  strength  of  mortars  made  from  such  sands. 
The  depth  of  color  may  be  measured  by  comparison  with  proper 
color  standards. 

Two  methods  of  procedure  have  been  developed:  (1)  For 
Field  Tests;  (2)  For  Laboratory  Tests. 


METHOD  FOR  FIELD  TESTS. 

Fill  a  12-oz.  graduated  prescription  bottle  to  the  4J-oz.  mark 
with  the  sand  to  be  tested.  Add  a  3%  solution  of  sodium  hydrox- 
ide until  the  volume  of  the  sand  and  solution,  after  shaking, 
amounts  to  7  oz.  Shake  thoroughly  and  let  stand  over  night. 
Observe  the  color  of  the  clear  supernatant  liquid. 

In  approximate  field  tests  it  is  not  necessary  to  make  com- 
parison with  color  standards.  If  the  clear  supernatant  liquid  is 
colorless,  or  has  a  light  yellow  color,  the  sand  may  be  considered 
satisfactory  in  so  far  as  organic  impurities  are  concerned.  On 
the  other  hand,  if  a  dark-colored  solution,  ranging  from  dark 

2 


reds  to  black  is  obtained  the  sand  should  be  rejected  or  used 
only  after  it  has  been  subjected  to  the  usual  mortar  strength 
tests. 

Field  tests  made  in  this  way  are  not  expected  to  give  quanti- 
tative results,  but  will  be  found  useful  in: 

(1)  Prospecting  for  sand  supplies; 

(2)  Checking  the  quality  of  sand  received  on  the  job; 

(3)  Preliminary  examination  of  sands  in  the  laboratory. 

An  approximate  volumetric  determination  of  the  silt  in  sand 
can  be  made  by  measuring  or  estimating  the  thickness  of  the 
layer  of  fine  material  which  settles  on  top  of  the  sand.  The 
per  cent  of  silt  by  volume  has  been  found  to  vary  from  1  to  2 
times  the  per  cent  by  weight. 


METHOD  FOR  LABORATORY  TESTS. 

To  a  200-gram  sample  of  dry  sand  add  100  cc.  of  a  3%  solution 
of  sodium  hydroxide  (NaOH)  and  digest  at  ordinary  tempera- 
ture, with  occasional  stirring,  for  24  hours.  Filter  this  solution 
through  a  good  grade  of  filter  paper ;  refilter  if  necessary.  The 
filtrate  must  be  clear.  Place  10  cc.  of  the  clear  filtrate  in  a  50  cc. 
Nessler  cylinder  and  dilute  to  50  cc.  with  distilled  water.  Shake 
thoroughly  and  let  stand  until  all  foam  and  bubbles  disappear. 
Determine  the  color  value  of  this  cylinder  by  comparing  it  with 
cylinders  containing  standard  solutions  of  alkaline  sodium  tan- 
nate.  Compare  the  colors  by  looking  through  the  full  depth  of 
the  solution  with  the  cylinders  held  toward  a  good  natural  light. 

Standard  Tannic  Acid  Solution  for  Color  Comparison.  The 
preparation  of  the  standard  tannic  acid  solution  for  comparing 
the  color  of  the  filtrate  should  be  begun  at  the  same  time  as 
the  treatment  of 'the  sand.  Add  10  cc.  of  a  2%  solution  of  tannic 
acid  in  10%  alcohol  to  90  cc.  of  a  3%  solution  of  sodium  hydrox- 
ide. The  sodium  hydroxide  combines  with  the  tannic  acid  to  form 
sodium  tannate.  Let  the  solution  stand  24  hours  at  room  tem- 
perature. Place  1,  2,  3,  4,  5,  6,  7,  8,  9  and  10  cc.,  respectively, 
of  this  solution  in  50  cc.  Nessler  cylinders  and  dilute  to  the  mark 
with  distilled  water.  The  amounts  of  tannic  acid  in  the  different 
cylinders  will  then  be  as  shown  in  the  following  table : 

Alkaline  Sodium 
Tannate  in  each 
cylinder— cc.  ...  123456789  10 

Tannic  acid  in 
each  cylinder  — 
milligrams 2  4  6  8  10  12  14  16  18  20 

Color  value  in 
parts  of  tannic 
acid  per  million 
of  sand  by  weight  100  200  300  400  500  600  700  800  900  1000 


It  is  desirable  to  have  good  sunlight  for  comparing  the  colors ; 
if  sunlight  is  not  available,  the  amount  of  tannic  acid  in  each 
of  the  cylinders  containing  the  standard  solutions  may  be  de- 
creased by  one-half  and  the  other  values  in  the  table  modified 
accordingly. 

In  case  the  solution  obtained  by  digesting  the  sand  with  the 
sodium  hydroxide  is  very  dark,  use  less  than  10  cc.  for  the  com- 
parison and  make  the  necessary  modifications  in  the  calculation 
of  the  color  values.  With  very  light-colored  solutions  use  more 
than  10  cc.  of  the  filtrate  for  the  comparisons.  The  depth  of 
color  of  the  solution  decreases  on  standing,  and  for  that  reason 
the  solution  should  be  made  up  fresh  for  each  day's  work. 

Method  of  Calculation.  An  example  will  make  clear  the  meth- 
od of  calculating  the  color  value  of  a  sand.  Suppose  that  10  cc. 
of  clear  filtrate  obtained  by  digesting  the  sand  with  100  cc.  of 
a  3%  solution  of  sodium  hydroxide  when  diluted  to  50  cc.  cor- 
responds in  color  to  the  Nessler  cylinder  containing  12  milligrams 
of  tannic  acid,  or  6  cc.  of  the  alkaline  tannate  solution.  The 
sand  will  then  have  a  color  value  of  600.  The  10  cc.  of  the  fil- 
trate placed  in  the  Nessler  cylinder  is  1/10  of  the  100  cc.  of  3% 
sodium  hydroxide  solution  which  was  added  to  the  sand,  and  the 
sample  of  sand  (200  grams)  is  1/5  of  a  kilogram;  therefore,  the 
milligrams  of  tame  acid  per  kilogram  of  the  sand,  by  weight, 
are  12  x  10  x  5  =  600 ;  or  the  tannic  acid  equivalent  when  ex- 
pressed in  parts  per  million  of  the  sand,  by  weight,  is  600. 

BASIS  OF  COLORIMETRIC  TEST. 

The  colorimetric  test  has  been  applied  to  sands  from  about 
40  widely  distributed  deposits  in  20  different  states.  The  research 
to  date  has  brought  out  the  following : 

1.  Examinations  of  deposits  of  defective  sands  show  that 
surface  loan  is  the  principal  sxmrce  of  contamination. 

2.  All  natural  sands  which  have  been  found  to  be  defective 
on  account  of  the  presence  of  organic  impurities  have  responded 
to  the  colorimetric  test  with  sodium  hydroxide,  and  all  sands 
which  have  given  high  color  values  have  shown  low  values  in 
mortar  tests. 

3.  Sands  which  were  similarly  graded  by  screening  out  and 
recombining  the  different  sizes  to  a  definite  sieve  analysis,  showed 
a  fairly  definite  relation  between  the  compressive  strengths  of 
1-3  mortars  at  7  and  28  days  and  the  color  values  of  the  sands. 

4.  The  motar-making  quality  of  sands  known  to   contain 
organic  impurities  has  been  much  improved  by  removing  the 
organic  matter,  either  by  repeatedly  digesting  them  with  sodium 
hydroxide  and  then  washing  free  from  alkali,  or  by  driving  off 
the  organic  impurities  by  ignition. 

5.  When  the  sodium  hydroxide  extracts  from  sands  which 
mortar  tests  had  shown  to  be  defective  were  purified  and  applied 
as  coatings  on  high-grade  sand,  that  sand  was  made  "defective" 
or  gave  much  reduced  mortar  strength. 


6.  It  is  impracticable  to  give  exact  values  for  the  relation 
between  the  color  value  of  a  sand  and  the  strength  of  mortars 
made  from  the  same  sand.  However,  the  tests  made  thus  far 
show  this  relation  to  be  about  as  follows : 

Reduction  in 

Color  Values  Compressive  Strength 

of  sand  of  1-3  Mortar 

Per  cent 

250  10-20 

500  15-30 

1000  20-40 

2000  25-50 

3000  30-60 

The  reduction  in  strength  is  based  on  compression  tests  at  ages 
of  7  days,  28  days,  and  3  months,  of  1-3  mortars  made  from 
the  same  sand,  before  and  after  coating  with  different  per  cents 
of  organic  impurities  which  had  been  extracted  from  defective 
sands  and  purified.  Tests  on  mortars  made  from  defective 
sands  as  received  and  after  removal  of  the  organic  impurities, 
either  by  repeated  extractions  with  sodium  hydroxide  or  by 
ignition,  showed  that  in  some  cases  the  reduction  in  strength  for 
a  given  color  value  was  even  greater  than  the  higher  values 
given  above.  Frequently  the  test  pieces  completely  disinte- 
grated in  the  storage  water.  The  higher  reductions  in  strength 
for  a  given  color  value  for  a  natural  sand  as  compared  with  the 
same  color  value  for  an  artifically  coated  sand  is  probably  due 
to  the  artificial  coating  being  more  easily  removed  by  the  sodium 
hydroxide. 

Sufficient  data  are  not  available  to  indicate  whether  or  not  the 
effect  of  a  given  quantity  of  organic  impurities  varies  with  the 
grading  of  the  sand.  The  reduction  in  strength  seems  to  de- 
crease slightly  with  the  age  of  the  test  pieces. 

APPARATUS. 

The  apparatus  required  for  making  the  colorimetric  test  of 
sands  will  vary  according  to  the  nature  of  the  investigation. 
Two  lists  are  given  below:  (1)  Apparatus  for  Field  Tests; 
(2)  Apparatus  for  Laboratory  Tests.  Sufficient  apparatus 
has  been  included  in  each  list  for  tests  on  5  samples  at  a  time. 

Approximate 
For  Field  Tests.  Cost 

5  12-oz.  graduated  prescription  bottles $    .25 

Stock  of  3%  solution  of  sodium  hydroxide  (dis- 
solve 1  oz.  of  sodium  hydroxide  in  enough 
water  to  make  32  oz.) 50 

$     .75 


For  Laboratory  Tests. 

15  Nessler  cylinders $  7.50 

5   3-in.  glass  funnels 1.00 

10   250-cc.  beakers 2.00 

1  test  tube  support 50 

100  qualitative  filter  papers,  11  cm.  diam.  (good 
grade) 50 

1  100-cc.  graduated  cylinder 75 

2  oz.  2%  solution  of  tannic  acid  in  10%  alcohol.       .50 
1  Ib.  sodium  hydroxide  (NaOH)   (sufficient  for 

150  tests)  50 

1  32-oz.  bottle  containing  3%  solution  sodium 

hydroxide 25 

1  10-cc.  Mohr 's  pipette,  graduated  in  1/10  cc. . .  .50 

$14.00 


CONCLUDING  REMARKS. 

Impurities  in  sands  other  than  organic,  such  as  clay  or  similar 
admixtures,  if  present  in  considerable  quantities,  may  be  ex- 
pected to  affect  the  strength  of  the  concrete.  However,  the 
examination  of  a  large  number  of  defective  sands  has  shown 
that  it  is  the  organic  impurities  of  a  humus  nature  which  are 
responsible  for  the  abnormally  low  strengths  of  such  sands.  The 
presence  of  these  impurities  can  be  detected  by  the  methods 
described  above. 

It  has  been  found  that  the  colorimetric  test  can  be  made  with 
a  greater  degree  of  uniformity  by  different  operators  than  can 
the  strength  tests  of  mortars  from  the  same  sands. 

The  fact  that  the  colors  produced  by  digesting  certain  natural 
sands  with  sodium  hydroxide  are  similar  to  those  produced  by 
treating  tannic  acid  in  the  same  manner  must  not  be  intepreted 
to  mean  that  the  organic  matter  in  sands  is  necessarily  tannic 
acid,  or  even  a  tannate.  The  chemical  compounds  which  make 
up  the  organic  material  are  probably  numerous  and  complicated. 

The  time  actually  required  for  a  single  determination  by  the 
laboratory  method  is  about  -J  hour.  If  tests  on  as  many  as  5 
samples  can  be  made  at  once,  the  total  time  consumed  need  not 
exceed  1  hour.  The  time  required  from  the  beginning  to  the 
end  of  the  test  is  about  24  hours. 

Inexperienced  operators  without  technical  training  or  previous 
experience  in  work  of  this  character  have '  secured  satisfactory 
results  at  their  first  trials. 

The  strength  test  of  mortar  is  used  as  the  criterion  of  the 
effect  of  impurities  in  sand.  In  making  strength  tests  of  sand 


mortars  and  interpreting  the  results  of  such  tests,  the  following 
points  require  particular  attention: 

(a)  Sampling  of  sand, 

(b)  Grading  of  sand, 

(c)  Quantity  of  water  used  in  mixing, 

(d)  Methods  of  mixing,  molding  and  testing, 

(e)  Form  and  size  of  test  piece. 

(a)  Great  care  is  necessary  to  insure  that  the  sample  tested 
is  representative  of  the  larger  lot  of  sand.     Good  judgment 
and  considerable  experience  are  necessary  to  secure  representa- 
tive samples    from  an    undeveloped    deposit.      Some    form    of 
sampler  should  be  used  in  the  laboratory  in  selecting  a  test  sample 
from  a  larger  lot. 

(b)  The  size  and  grading  has  an  important  influence  on 
the  concrete  and  mortar-making  qualities  of  sands.     A  sand 
graded  from  coarse  to  fine,  with  the  coarse  particles  up  to  about 
j  in.  in  diameter,  is  desirable  for  use  in  concrete.    Fine  sands 
when  mixed  to  the  same  plasticity,  give  mortar  strengths  much 
below  those  of  coarse,  well-graded  sands.    The  influence  of  the 
grading  of  sand  on  the  strength  of  the  mortar  must  not  be  con- 
fused with  the  effect  of  impurities. 

(c)  The  quantity  of  water  used  in  mortar  tests  has  just  as 
important  an  influence  on  the  strength  as  the  amount  of  ce- 
ment.    The  quantity  of  water  required  for  normal  consistency 
of  mortar  made  from  a  given  cement  is  a  function  of  the  grading 
of  the  sand. 

(d)  The  methods  used  in  mixing,  molding  the  test  pieces, 
storing  and  testing  the  specimens,  etc.,  used  in  standard  tests 
of  cement  should  be  followed  in  making  strength  tests  of  sand 
mortars. 

(e)  Compression  tests  of  mortar  are  believed  to  be  more 
significant  than  tension  tests.     A  2  by  4-in.  cylinder  has  been 
found  to  give  satisfactory  results  as  a  compression  test  piece. 
This  is  the  form  of  test  piece  recommended  by  the  American 
Society  for  Testing  Materials  in  their  proposed  tentative  speci- 
fication for  compression  tests  of  cement. 


The  funds  for  this  investigation  were  furnished  by  Committee 
C-9  on  Concrete  and  Concrete  Aggregates,  of  the  American  So- 
ciety for  Testing  Materials,  Sanford  E.  Thompson,  Boston,  Mass., 
Chairman,  F.  W.  Kelley,  Albany,  N.  Y.,  Chairman  of  Sub- Com- 
mittee V,  on  Impurities  Affecting  Fine  Aggregates  for  Concrete. 

The  study  of  impurities  in  natural  sands  is  being  continued 
as  a  part  of  the  work  of  the  Structural  Materials  Research 
Laboratory,  Lewis  Institute,  Chicago.  Special  attention  will 
be  given  to  further  improvements  in  the  colorimetric  test,  and 
to  discovering  remedial  measures  which  can  be  used  in  a  com- 
merical  way  for  counteracting  the  effects  of  such  impurities. 


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2   1933 


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